Low temperature properties such as superconductivity are now widely used in a range of different applications including Magnetic Resonance Imaging (MRI), superconducting magnets, sensors and in fundamental research. Historically, the evaporation of cryogenic liquids such as nitrogen or helium has been used as a cooling mechanism in order to reach the low temperatures required for such applications. Cryogenic liquids, particularly helium, have associated disadvantages in that they are often “consumable” due to incomplete recovery of boiled off gas. Furthermore such apparatus for storing or otherwise handling cryogenic liquids is often bulky and requires special handling procedures. Such apparatus and procedures are somewhat incompatible with patient care environments.
More recently, pulse tube refrigerators (PTRs) have been used to replace cryogenic liquids in providing an alternative refrigeration mechanism. In contrast with the evaporation of cryogenic liquids, PTRs do not rely upon a phase change within the coolant. Instead, PTRs operate upon a principle of using the cooling which is associated with the work of compression and expansion of a working gas coolant such as helium. Accordingly, the use of PTR systems is of particular interest for cooling apparatus for medical applications such as MRI systems.
PTRs provide cooling of a cold stage at relatively modest cooling powers of a few Watts, to temperatures below 4 Kelvin. These low temperatures are produced by expanding and compressing the working gas in a thermodynamic cycle. In order to run the cycle, a typical PTR system comprises three major components—a compressor, a valve assembly and a pedestal part. The compressor supplies the cryocooler with high pressure compressed gas such as helium via a high pressure line, and receives gas back from the cryocooler in a low pressure line. The pedestal part comprises pulse tube(s), regenerator tube(s) comprising different regenerator materials for heat exchange with the incoming and outgoing gas where the cooling power is supplied, and the cooled stage(s) thermally connected to the subject to be cooled. The valve assembly connects the high and low pressure sides of the compressor to the pulse tubes and regenerators within the pedestal part, and controls the timing and distribution of gas flows between the compressor and pedestal part in order to effect the thermodynamic cycle and subsequent cooling.
A major advantage of PTRs is that they have very few moving parts in contrast to alternative refrigerators such as Gifford McMahon coolers, and this makes them particularly beneficial for applications where low levels of vibration noise are needed. Examples of such applications include MRI imaging where, in addition to improving image resolution, there is a desire to reduce vibrations and audible noise since it is well established that many MRI procedures are aborted by patients when they become distressed during such procedures. Furthermore, the vibrations from a PTR cold head which is in close contact with a cryostat may cause cyclical vibrations within the MRI magnetic field. This may adversely affect the quality of the MRI data. Other applications for PTRs include refrigerators going to very low temperatures where vibrations are a source of heat generation and sensitive optical, magnetic or electronic experiments where vibrations disturb the measurements and additional work is required to reduce the vibration levels.
However, the cyclical flow of gas within the pedestal of a PTR produces a large range of pressures within the tubes of the pedestal, typically ranging from approximately 5 bar to 28 bar. These pressure changes cause expansion and contraction of the pedestal tubes due to the elasticity of the construction materials, and these are transferred as vibrations to the cold stage, which can typically be in the range of 10-50 microns.
Attempts to reduce the vibrations of the cold stage have focussed on increasing the rigidity of the tubes in the pedestal. For example, JP-A-2003329323 discloses a two stage PTR where the wall thickness of a second (colder) stage pulse tube is set to be larger than the wall thickness of a first stage pulse tube in order to increase its rigidity. This increase in rigidity decreases the vibration amplitude in the second cold stage.
US-A-2008/0173026 uses a variable wall thickness in the tubes of the pedestal part, with the tubes being thicker at the higher temperature end in order to increase their rigidity and therefore reduce vibration, and thinner at the low temperature end so as to minimise the decrease in cooling performance. The walls are either stepped or continuously sloped from the thicker high temperature part to the thinner low temperature part.
However, due to the importance and widespread use of pulse tube refrigerators, there is a continued need to reduce the vibrations present in such refrigerators.